Embedded magnetic random access memory (MRAM)

ABSTRACT

A magnetic random access memory (MRAM) cell includes an embedded MRAM and an access transistor. The embedded MRAM is formed on a number of metal-interposed-in-interlayer dielectric (ILD) layers, which each include metal dispersed there through and are formed on top of the access transistor. A magneto tunnel junction (MTJ) is formed on top of a metal formed in the ILD layers that is in close proximity to a bit line. An MTJ mask is used to pattern the MTJ and is etched to expose the MTJ. Ultimately, metal is formed on top of the bit line and extended to contact the MTJ.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 13/623,054, filed on Sep. 19, 2012, by Parviz Keshtbod, andentitled “EMBEDDED MAGNETIC RANDOM ACCESS MEMORY (MRAM)”, which is adivisional application of U.S. patent application Ser. No. 12/778,725,filed on May 12, 2010, by Parviz Keshtbod, et al., and entitled“Embedded Magnetic Random Access Memory (MRAM)”, which claims priorityto a previously-filed U.S. Provisional Application No. 61/177,641entitled “Embedded Magnetic Random Access Memory (MRAM)”, filed on May12, 2009 and is a continuation-in-part of U.S. patent application Ser.No. 11/776,692, entitled “Non-Volatile Magnetic Memory Element withGraded Layer”, filed on Jul. 12, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to non-volatile magnetic memory andparticularly to embedded magnetic random access memory (MRAM).

2. Description of the Prior Art

Magnetic random access memory (MRAM) has gained much notoriety in therecent decade and is expected to ultimately replace non-magnetic memoryand even though, it is non-volatile memory itself, it is furtherexpected to replace currently-employed non-volatile memory such as flashand EEPROM. Other expected applications include replacement of diskdrives.

One of the problems currently impeding the growth of MRAM is the size ofMRAM cells, which are large enough to make them undesirable for manyapplications requiring small form factor. A typical MRAM cell includes amagnetic tunnel junction (MTJ) and an access transistor used for readingfrom the MTJ and writing to the MTJ. The access transistor is typicallymade using known non-magnetic processes, such as CMOS, and obviously,MTJs are made of processes unknown to regular circuitry process such asCMOS. The combination of the foregoing results in large memory cells.Moreover, current manufacturing techniques build the MTJ on top of allcircuitry making the cell higher or taller than desired. The largenumber of such memory cells used in a magnetic memory array makes forlarger than desired arrays that are not readily practical for use invarious applications.

A number of specific applications benefiting from small MRAM cell sizesare now discussed. Most large random logic circuits, including microprocessors need fast and easily accessible memory for scratch pad. Thisneed is mostly satisfied with static RAMs. Static RAMs are volatile fastrandom access memories with large cell sizes. The reason for a largecell size is that they use 6 transistors per cell, with two of thep-channels, situated inside the n-well. This n-well makes the SRAM cellfairly large. In the older days, system designers used static RAM chips(or integrated circuits), which were mounted on printed boards next tothe micro processors. Since the processors have experiencedextraordinary growth in popularity over the past several decades and dueto an increase in their size, more SRAM is needed on the circuit boards.

These days, processors function much faster and if data has to travelfrom SRAM to a logic circuit through the board, band width shrinks andhigh frequency requirements can not be met. Circuit designers prefer toreplace SRAM with some thing smaller and faster. DRAMs and Flashmemories are generally too slow. To fulfill these requirements, thelogic designers place SRAM next to the logic circuit on the samesemiconductor device. In this manner, they design all the memory theyneed on the same chip next to the random logic, and such close proximitysatisfies the speed requirements. The main problems with this approachare that 1) the large size of SRAM cells makes the processor large andexpensive; and 2) SRAM is volatile and all the information stored in theSRAM which controls the functions of the micro processor is lost withloss of power. To safe guard against such data loss, all the data istypically stored on Flash memory or a hard drive, and it can be loadedinto the SRAM from Flash or hard disk.

What is needed is a magnetic random access memory cell including amagnetic tunnel junction and access transistor that is small in formfactor.

SUMMARY OF THE INVENTION

To overcome the limitations in the prior art described above, and toovercome other limitations that will become apparent upon reading andunderstanding the present specification, the present invention disclosesa method and a corresponding structure for a magnetic storage memorydevice that is based on spin current-induced-magnetization-switchinghaving reduced switching current in the magnetic memory.

Briefly, an embodiment and method of the present invention includes amagnetic random access memory (MRAM) cell having an embedded MRAM. TheMRAM cell includes an access transistor, made from non-magneticcircuitry, and an embedded MRAM by forming a number ofmetal-interposed-in-interlayer dielectric (ILD) layers, which eachinclude metal dispersed therethrough and are formed on top of the accesstransistor. An magneto tunnel junction (MTJ) is formed on top of a metalformed in the ILD layers that is in close proximity to a bit line. AnMTJ mask is used to pattern the MTJ and is etched to expose the MTJ.Ultimately, metal is formed on top of the bit line and extended tocontact the MTJ.

In another embodiment and method, a hard-to-etch metal layer is used todefine a MRAM area between two sets of ILD layers in which the MTJ isbuilt.

These and other objects and advantages of the present invention will nodoubt become apparent to those skilled in the art after having read thefollowing detailed description of the preferred embodiments illustratedin the several figures of the drawing.

IN THE DRAWINGS

FIGS. 1( a) through 1(g) show a substrate undergoing the formation ofone or more MTJs and non-magnetic circuits having five metal interposedwith interlayer dielectric (ILD) layers formed thereon, in accordancewith a method and apparatus of the present invention.

FIGS. 2( a) through 2(k) show a substrate undergoing the formation ofone or more MTJs and non-magnetic circuits having nine metal interposedwith interlayer dielectric (ILD) layers formed thereon, in accordancewith a method and apparatus of the present invention.

FIG. 3 shows a top view of the layout 202 of a portion of MRAM 100 wherean MRAM cell comprising access transistor and MTJ is formed, inaccordance with an embodiment of the present invention.

FIG. 4 shows a cross section view of the layout 202. M3, which is thebit line 21 of FIG. 2( e) is shown coupled to MTJ 60 (or 302) throughmetal 64.

FIG. 5 shows a schematic of a MRAM cell including an MTJ (shown as avariable resistor) and an access transistor.

FIG. 6 shows a cross section of a portion of a magnetic memory arraycomprising two embedded MRAMs, in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description of the embodiments, reference is made tothe accompanying drawings that form a part hereof, and in which is shownby way of illustration of the specific embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized because structural changes may be madewithout departing from the scope of the present invention. It should benoted that the figures discussed herein are not drawn to scale andthicknesses of lines are not indicative of actual sizes.

In accordance with an embodiment and method of the present invention,magnetic memory, such as magnetic random access memory (MRAM) isembedded into non-magnetic circuits to make a die formed on a wafer ofmultiple layers. In one embodiment of the present invention, MRAM isbuilt on top of all of the metal layers that are formed on top of asubstrate. Specifically, an MRAM cell is formed on the top-most metallayer and a metal (or contact). Subsequently, metal is used to connectthe MRAM cell to another metal that may be a bit line, for example. Inthis manner, there is no interlayer dielectric (ILD) deposition afterMTJs are formed thereby avoiding unnecessary exposure of MRAM cells toharmful environments such as high temperatures. It is important to notethat MRAM is embedded in non-memory circuitry after the non-memorycircuitry is formed. The non-memory circuitry is made using processessuch as but not limited to complementary metal-oxide-semiconductor(CMOS), BiCMOS, or Bipolar.

In another embodiment of the present invention, MRAM is used in additionto non-magnetic random access memory by placing a hard-to-etch metallayer over an area that is to be used for forming MRAM. In oneembodiment, this hard-to-etch metal layer is made of Tantalum (Ta) orTungsten (W) or any other metal with a lower etch rate than that of ILD.It is advantageous to place the MRAM on metal layers up to where themetal layers have the smallest design rule so that a smaller MRAM cellmay be formed. The non-magnetic circuitry, which lies outside of thearea preserved for MRAM, is advantageously built using the standardprocess used for CMOS manufacturing. Prior to formation of MRAM,interlayer dielectric (ILD) is covered by a hard-to-etch metal layer.This hard-to-etch metal layer serves as a stop layer to ensurepreservation of the area to be used for MRAM. Subsequently, the stoplayer is removed exposing a metal layer on top of which the MTJ isdeposited. Finally, another metal, such as but not limited to tantalum(Ta) is deposited and is used to connect the top of the MTJ to a metallayer that forms the bit line. In this manner, the bit line is connectedto the top of the MTJ. Forming MRAM on a metal layer closest to thesubstrate or farthest from the top-most metal layer advantageouslyallows the size of an MRAM to be smaller than that experienced bytechniques that form the MRAM on the top most metal layer oftentimesoccurring due to loose design rules associated with the top most metallayers.

In an exemplary embodiment, where 9 metal layers are employed, amagnetic memory cell (or MTJ) that is formed on the third metal layerfrom the wafer is about 6-10 times smaller than it would be if it wereto be formed on the top most metal layer of the substrate. The size ofthe MTJ is important in that the larger the MTJ, the more real estateconsumed and the higher the switching current required to switch thestates of a MTJ from one magnetic state (or logic state) to another.

On the other hand, forming the MTJ on the bottom-most metal layer causesthe MTJ to be exposed to undesirable non-magnetic manufacturingprocesses such as high temperatures. MTJ characteristics change whenexposed to temperatures higher than 350 degrees Celsius and particularlywhere the exposure is many times over. Such exposure would take place ifthe MTJ was made on top of the bottom-most metal layer and the waferswere processed to form all other metal layers. More specifically,exposure to higher temperatures would degrade the MTJ and reduce thedifference between its high and low resistances as defined by atunneling magnetic resistance (TMR) value commonly known to representthe difference between the highest resistance of that the minus thelowest resistance that the MTJ divided by the lowest resistance that theMTJ. Placing the MTJ in such early stage of the process would expose theMTJ resistor to all high temperature processing of ILD depositionsbetween metal layers. In most cases the deposition temperatures of theILD layers are around 450 C.

In another embodiment and method of the present invention, MRAM, whichincludes an MTJ, is formed on the top most metal layer.

FIGS. 1( a) through 1(g) show a substrate on top of which is formed oneor more MTJs of magnetic random access memory (MRAM) and non-magneticcircuitry having five metal interposed with ILD layer formed thereon, inaccordance with a method and apparatus of the present invention. As willbe apparent shortly, the MTJs, each one of which along with an accesstransistor defines a MRAM cell, are embedded in the non-magneticcircuit. Accordingly, CMOS circuits are built in accordance withstandard processes and the MRAM formation is not exposed to suchprocesses thereby avoiding those that may be too harsh for it, such ashigh temperatures.

Referring now to FIG. 1( a), an embedded magnetic random access memory(MRAM) 100 is shown to include diffusion regions 102, transistor gates104, metal-interposed-in-interlayer dielectric (ILD) layer 124,metal-interposed-in-interlayer dielectric (ILD) layer 126,metal-interposed-in-interlayer dielectric (ILD) layer 128,metal-interposed-in-interlayer dielectric (ILD) layer 130 andmetal-interposed-in-interlayer dielectric (ILD) layer 132, metal (orcontacts) 106, ILD 134, ILD 136, ILD 138, ILD 140, and ILD 142. MRAM 100integrates magnetic memory and non-magnetic circuits, which may includenon-magnetic memory.

The diffusion regions 102 may be N+ doped or P+ doped, as well known inthe art. The diffusion regions 102 are formed on top of the siliconsubstrate 122. The transistor gates 104, which are also known in the artand together with the diffusion regions form a part of the non-magneticcircuitry, are formed on top of the substrate 122. The non-magneticcircuitry 101 is shown on either side of an area where magnetic memoryis to be formed. Metal 108 is a bit line that is eventually connected toa MTJ.

Layers 134, 136, 138, 140 and 142 are each made using the well knowndouble damascene process where metal is deposited over and intoseparations between the ILD and chemically-mechanically polished (CMPs)to form the layer with ILD separated by metal. For example, layer 134has metal 106 dispersed throughout. Similarly, layer 136 is separated bymetal 106 throughout as are each of the layers 138, 140 and 142. Layer134 and the metal 106 collectively comprise metal interposed with ILDlayer 124 and the layer 136 and metal 106 collectively comprise metalinterposed with ILD layer 126 and so on through layer 142.

It is understood that the figures included and discussed herein are notdrawn to scale. For example, metal 106 may be larger in width than thatwhich appears in the figures.

At a subsequent processing step of MRAM 100, shown in FIG. 1( b), a MTJ112 is then formed on top of one of one of the metals 106 that islocated within close proximity of bit line 108. This metal (106) is thedrain of an access transistor coupled to the MTJ 112 upon formation ofthe MTJ 112. It is noted that the MTJ 112 is shown to not be necessarilyaligned with and rather offset from the metal 106 that is directlybeneath it to emphasize the point that such an alignment isadvantageously unnecessarily although it is certainly contemplated. Thealignment of an MTJ relative to the metal is a challenge presented incurrent manufacturing designs. In the various embodiments of the presentinvention, such an alignment is unnecessary such that the MTJ 112 is notnecessarily entirely formed on top of the metal 106. The formation ofthe MTJ 112 on top of the metal 106 merely requires that the MTJ contactthe metal 106 enough to allow sufficient current to flow through the MTJ112. The diffusion regions 102 shown in FIG. 1( b) to be formed underand the left of the MTJ 112 and the transistor gate 104 shown formedthere between collectively form an access transistor. It is understoodthat bit line 108 is coupled to a transistor on the substrate 122.

Next, as shown in FIG. 1( b), an MTJ mask 110 is formed on top of MTJ112 to pattern the MTJ 112. The MTJ mask 110 is made of photoresist inone embodiment of the present invention and in another embodiment, it ismade of hard mask. Next, in FIG. 1( c), the photoresist of the MTJ mask110 is removed by a process of oxygen ashing (using oxygen plasma), wellknown in the art. Upon removal of the MTJ mask 110, the MTJ 112 isexposed. The purpose of the MTJ mask 110 is to protect the MTJ 112 frombeing etched when the surface of the wafer onto which the MRAM cell isformed is etched. Next, as shown in FIG. 1( d), spacer 116 is formed ontop and around MTJ 112 by depositing spacer 116 onto the wafer thatincludes substrate 122 with the MRAM being formed on top thereof. In oneembodiment of the present invention, the spacer 116 is formed bydepositing silicon nitride (Si₃N₄) on top of surface of substrate 122 oron top of all layers formed on top of the substrate 122 up until thispoint, as discussed above, and then etching to form the spacer 116. Inanother embodiment, spacer 116 is made in the same manner using oxideinstead of silicon nitride.

In one embodiment, the deposited spacer 116 (either silicon nitride oroxide or any other suitable material) is approximately 1,500 Angstromsin thickness. Next, blanket etching is performed on the silicon nitrideand the remainder of the surface of the substrate to form a nitridespacer all around and top of the MTJ 112.

Next, at the step in FIG. 1( e), metal 118 is deposited on top of thebit line 108 and a portion of the top of the layer 142 extending throughthe entire top of the MTJ 112 thereby connecting the bit line 108 to thetop of the MTJ 112. In one embodiment, metal 118 is made of Ta or W orany other metal that can tolerate temperatures around 350-400 degreesCelsius. In one embodiment, metal 118 has a thickness of approximately1,500 Angstroms. The foregoing step causes the bit line 108 to beconnected to MTJ 112, which is coupled to diffusion region 102 throughthe metal 106. The diffusion region 102, which in one embodiment is a N+diffusion, is the drain of the access transistor coupled to MTJ 112whose transistor gate is 104. Next, an interconnect mask 109 is formedon top of metal 118, to define an interconnect, as shown in FIG. 1( f).The interconnect mask 109 is removed by etching it. Next, passivationlayers 107 are deposited at temperatures at or below 350 degreesCelsius. Passivation layers 107 are deposited on top of the wafer thatincludes substrate 122 (on top of which the MRAM cell is being formed),with the passivation layers covering the MRAM 100. The passivationlayers are then masked and etched, as shown in FIG. 1( g). The purposeof the passivation layers is to protect the layers underneath fromoxidation, contaminating, scratching or any other type of damage thatmay be experienced by the wafer. The interconnect mask 109 is typicallymade of photoresist and is ultimately removed by ashing. Upon theremoval of the interconnect mask 109, the MTJ 112, which is embedded inthe CMOS formed on the surface of the substrate 122 is exposed alongwith the metal 118 and bit line 108.

FIGS. 2( a) through 2(k) show a silicon substrate (included in a wafer),similar to the silicon substrate 122, undergoing the formation of one ormore MTJs and CMOS circuits having nine metal interposed with ILD layersformed thereon, in accordance with a method and apparatus of the presentinvention.

Referring now to FIG. 2( a), an embedded magnetic random access memory(MRAM) 10 is shown to include diffusion regions 12, transistor gates 14,metal interposed with interlayer dielectric (ILD) layer 16, metalinterposed with ILD layer 18, metal interposed with ILD layer 20, metal(or contacts) 22 (one of which is bit line 21), ILD 24, ILD 26, ILD 28,and hard-to-etch metal layer 30. The diffusion regions 12 may be N+doped or P+ doped, as well known in the art. The diffusion regions 12are formed on top of the silicon substrate 31. The transistor gates 14,which are also known in the art and together with the diffusion regionsform a part of the CMOS circuitry, are formed on top of the substrate31.

It is understood that bit line 21 is coupled to a transistor on thesilicon substrate 31.

Metal interposed with ILD layer 16, metal interposed with ILD layer 18and metal interposed with ILD layer 20 are each made using the wellknown double damascene process where metal is deposited over and intocuts between the ILD and chemically-mechanically polished (CMPs) to thesame level as the ILD. For example, ILD 24 is etched and is filled withmetal 22 throughout. Similarly, ILD 26 is separated by metal 22throughout as is ILD 28. ILD 24 and the metal 22 collectively comprisemetal layer interposed with ILD 16 and the ILD 26 and metal 22collectively comprise metal layer interposed with ILD 18 and ILD 28 andmetal 22 collectively comprise metal layer interposed with ILD 20. Asshown in FIG. 2( a), three layers, 16, 18, 20 are formed prior to theformation of hard-to-etch metal layer 30, which is shown formed on topof metal layer interposed with ILD 20.

In some embodiments, a nitride layer 111 is optionally deposited on topof metal interposed with ILD layer 20 and the hard-to-etch metal layer30 is then deposited on top of the nitride layer, which acts as acushion when the layer 30 is ultimately etched.

In some embodiments, a well known double-damascene process is used toform the layers 16, 18, 20, 40, 42, 46 and 48. Other processes arecontemplated.

It is understood that the figures included and discussed herein are notdrawn to scale. For example, metal 22 may be larger in width than thatwhich appears in the figures.

In one embodiment, hard-to-etch metal layer 30 is made of Tantalum (Ta)or Tungsten (W). In other embodiments, hard-to-etch metal layer 30 ismade of any metal having an etch rate, during ILD etching, that is about5×-8× lower than the rate of etching of the ILD. In one embodiment, thethickness of layer 30 is about 1000 to 1,500 Angstroms.

At a subsequent processing step of MRAM 10, shown in FIG. 2( b), a MRAMarea definition mask 32 is deposited on top of hard-to-etch metal layer30 but only in a predetermined area where MRAM is to be formed and ontop of at least two metal lines with at least one metal line having adiffusion region 12 formed there-under. In the embodiment of FIG. 2( b),the two metal lines are metal 22 and bit line 21. This is importantbecause, as will be evident in subsequent figures, one of the at leasttwo metal lines that is not formed on top of a diffusion region is usedas a bit line, in FIGS. 2( a) through 2(g), this is bit line 21, and theother metal 22 that is formed on top of the diffusion region is thedrain of the access transistor that will be coupled to an MTJ of theembedded MRAM. The area of mask 32 essentially and approximately definesthe area in which MRAM is to be formed. The process of usingphoto-resist mask is well known.

MRAM area definition mask 32 serves to protect an area under it forformation of MRAM while the areas on either side of it, of which thetop-most layer is hard-to-etch metal layer 30, are exposed. CMOScircuitry is formed underneath of both MRAM area definition mask 32 andthe exposed part of hard-to-etch metal layer 30. The hard-to-etch metallayer 30 is covered in the MRAM area by the MRAM area definition mask 32and the rest is etched. Thus, only the MRAM area is then covered withhard-to-etch metal layer 30. The MRAM area definition mask 32 is removedby etching some time prior to the formation of the IDL layers 40-48.

As will be discussed in further detail shortly, the wafer is thenprocessed according to known techniques until all the metals are done.Then, a reverse of the mask 32 is used to etch all the ILDs on top ofthe remaining hard-to-etch metal layer 30. Next, the foregoing remaininghard-to-etch metal layer 30 is etched the MTJ is deposited and formed.Spacer formation, as discussed and shown relative to FIGS. 1( a)-1(g)above, follows and then the top of the MTJ is connected to the bit linevia a tungsten (W) or tantalum (Ta) strap.

From the process of FIG. 2( b), the process then proceeds to forming theremaining circuitry 38 around the hard-to-etch metal layer 30, as shownin FIG. 2( c). Additional metal layer interposed with ILDs (ILD layers40-48), forming a part of remaining circuitry 38 are formed on top ofmetal layer interposed with ILD 20. The ILD layer 40 is formed on top ofthe ILD layer 20 as well as on top of the hard-to-etch metal layer 30 inthe area on top of the ILD layer 20 where the hard-to-etch metal layer30 exists. In the embodiment of FIG. 2( c), nine layers of metal layerinterposed with ILD are shown formed, however, in other embodiments, oneor more number of metal layer interposed with ILD layers may beemployed.

The last or top-most metal layer interposed with ILD to be formed, inthe embodiment of FIG. 2( c) is metal layer interposed with ILD layer48. The metal layer interposed with ILD between metal layer interposedwith ILD layer 20 and metal layer interposed with ILD layer 48 are metallayer interposed with ILD layers 40, 42, 44, 46 and 48. On top of metallayer interposed with ILD layer 20 is formed metal layer interposed withILD layer 40 and on top of metal layer interposed with ILD layer 40 isformed metal layer interposed with ILD layer 42. On top of metal layerinterposed with ILD layer 42 is formed metal layer interposed with ILDlayer 44 and on top of metal layer interposed with ILD 44 is formedmetal layer interposed with ILD layer 46 on top of which is formed metallayer interposed with ILD layer 48. Subsequently, photoresist layer 34is deposited on an adjacent side where an MRAM is to be formed and topof metal layer interposed with ILD layer 48 and photoresist layer 36 isformed on an opposing adjacent side where an MRAM is to be formed ofmetal layer interposed with ILD layer 48 and on top of metal layerinterposed with ILD layer 48. The photoresist layers 36 and 34 do notspan the entire top of metal layer interposed with ILD 48 and rather aresubstantially separated from each other by a space defined by the lengthof hard-to-etch metal layer 30 although they each slightly overlap witha respective end of hard-to-etch metal layer 30. The photoresist layers34 and 36 may be collectively considered a single layer deposited on topof the ILD layer 48 except that it is not deposited on the MRAM area. Itis understood that while the embodiments disclosed herein use a specificnumber of ILD layers, any number of such layers may be employed assuited for the particular intended application.

Photoresist layers 34 and 36 serve to protect the materials under themfrom being etched thereby acting as a mask leaving exposed the area ontop of metal layer interposed with ILD layer 48 where no photoresist hasbeen deposited. Next, as shown in FIG. 2( d), etching is performed onthe surface of the wafer onto which the MRAM cell is formed to removethe hard-to-etch metal layer 30 but in the process of so doing, thephotoresist layers 34 and 36 are also etched but because the thicknessof each of the photoresist layers 34 and 36 is larger than the thicknessof the hard-to-etch metal layer 30, a part of the photoresist layers 34and 36 remains after etching whereas substantially all of thehard-to-etch metal layer 30 is removed after etching. In an exemplarymethod and embodiment, reactive ion etching (RIE) is utilized althoughother known techniques are contemplated.

During the etching step of FIG. 2( d), the materials below photoresistlayers 34 and 36 remain while materials above hard-to-etch metal layer30 are removed. Layer 30 protects and leaves in tact the material formedbelow it, as it protects these lower layers from being etched. Layer 30extends, on either end thereof, to slightly overlap with each of thephotoresist layers 34 and 36. These extended areas serve to avoidetching the remaining ILDs. This overlap occurs because when the layers34 and 36 are formed, they are intentionally formed to slightly overlapor extend over a respective end of the layer 30 where each of the layers34 and 36 is located. For example, during formation, the layer 34 isformed to slightly overlap with the layer 30 at an end of the layer 30that is closest to the layer 34 and similarly, the layer 36 is formed toslightly overlap with the layer 30 at an end of the layer 30 that isclosest to the layer 36.

The void that is left by the substantial removal of materials that wereon top hard-to-etch metal layer 30 and between layers 34 and 36 isdefined as MRAM area 50.

Next, an etching process is employed as shown in FIG. 2( e) to removehard-to-etch metal layer 30. While this process removes most of thehard-to-etch metal layer 30, it does leave behind those parts ofhard-to-etch metal layer 30 that are close to the end-sides of MRAM area50. MTJ 60 is then formed substantially on top of one of the metals 22that is located under the MRAM area 50, as discussed hereinabove wherethe MRAM area definition mask 32 was stated to essentially define theMRAM area 50. As with MTJ 112, MTJ 60 need not be directly aligned withthe metal 22 and can rather be offset therefrom for ease ofmanufacturing.

Next, as shown in FIG. 2( f), spacer 62 is formed on top and around MTJ60 by depositing spacer 62 on top of the MTJ 60. In one embodiment,spacer 62 is made of silicon nitride (Si₃N₄) or oxide. In oneembodiment, the deposited silicon nitride is approximately 1,500Angstroms in thickness.

Next, as shown in FIG. 2( f), blanket etching is performed on the spacer62 to form a nitride spacer 113 all around the MTJ 60, as shown in FIG.2( g). The shape of the spacer 113 is similar to a dome structurebecause of the MTJ 60.

Next, at the step shown in FIG. 2( h), metal 64 is deposited to connectthe bit line 21 to the top of the MTJ 60 by extending from the bit line21 and on top of the ILD layer 28 to and through the top of the MTJ 60.In one embodiment, metal 64 is made of Ta or W or any other metal thatcan tolerate temperatures around 350-400 degrees Celsius. In oneembodiment, metal 64 has a thickness of approximately 1,500 Angstroms.Metal 64 is a bit line that is now connected to MTJ 60. MTJ 60 iscoupled to diffusion region 12, which is the drain of an accesstransistor coupled to MTJ 60 whose transistor gate is 14 (or 65).

Next, as shown in FIG. 2( i), an interconnect mask 117 is formed on topof metal 64, to define the interconnect. The interconnect mask 117 isremoved by etching leaving behind the MTJ untarnished. Next, passivationlayers 119 are deposited on top of layers built on top of the substrate31 up to now, at temperatures at or below 350 degrees Celsius.Passivation layers 119 are then masked and etched, as shown in FIG. 2(k). The interconnect is subsequently removed.

FIG. 3 shows a top view of the layout 202 of a portion of MRAM 100 wherean MRAM cell comprising an access transistor and an MTJ is formed, inaccordance with an embodiment of the present invention.

In FIG. 3, metal 204 is analogous to metal 64 connecting bit line 200,which is analogous to the bit line 21 of FIG. 2( e), to a correspondingMTJ. This MTJ connects to the drain of the access transistor thoughcontact 206, which is a metal. Gate 208 is the gate of the accesstransistor and is coupled to the word line (WL 306 in FIG. 4). Sourceline (SL) 210, which is SL 308 in FIG. 4, serves as the source of theaccess transistor. Metal 212 is M1 (or M2) is coupled to the source line210.

The size of the MRAM cell shown in FIG. 3 is effectively 19.5 F², “F”representing the minimum feature size. This cell size is approximately3-4 times smaller than current prior art MRAM cell sizes. The minimumcell length of a MRAM cell of an embodiment of the present invention is0.5+1+1+0.75 or 3.25 F and its minimum cell width is 1.5+1.5+1.5+1.5 or6 F and its minimum transistor width is 3.5 F. It is understood thatthis is an exemplary MRAM cell size and that other sizes arecontemplated.

FIG. 4 shows a cross section view of the layout 202. M3, which is thebit line 21 of FIG. 2( e) is shown coupled to MTJ 60 (or 302) throughmetal 64. MTJ 302 is shown formed on top of three layers, as previouslydiscussed, and coupled to the drain 310 of access transistor 304. Thegate of transistor 304 is shown coupled to the word line 306 and thesource of the transistor 304 is shown coupled to the source line 308.

FIG. 5 shows a schematic of a MRAM cell including an MTJ (shown as avariable resistor) and an access transistor.

FIG. 6 shows a cross section of a portion of a magnetic memory arraycomprising two embedded MRAMs, in accordance with an embodiment of thepresent invention. More specifically, in FIG. 6, a magnetic memory array600 is shown to include a bit line (BL) 602, BL 604, sense line (SL)630, P substrate 618, N+ diffusion regions 612, 614 and 616, word line(WL) 610 and WL 608 and circuits 620 and 624. The circuits 620 and 624include non-magnetic circuitry, such as CMOS circuits and comprise theaccess transistors used to access the embedded MRAMs. That is, an accesstransistor, included in the circuitry 620, is formed on the N+ diffusionregion 612. The N+ diffusion region 612 is formed in the P substrate618, as are N+ diffusion regions 614 and 616. The access transistor inthe circuitry 620 is coupled to an MRAM 626, which is shown formed ontop of the circuitry 620 and coupled thereto through the metals and viasM1, M2, M3, Via-1 and Via-2 shown positioned between the circuitry 620and the MRAM 626. The BL 602 is formed on top of the MRAM 626, asdiscussed relative to prior embodiments herein. While not shown in FIG.6 due to the nature of the figure being a cross section view, the WL 610is coupled to the access transistor in the circuitry 620. Similarly, anaccess transistor, included in the circuitry 624, is formed on the N+diffusion region 616. The access transistor in the circuitry 624 iscoupled to an MRAM 628, which is shown formed on top of the circuitry624 and coupled thereto through the metals and vias M1, M2, M3, Via-1and Via-2 shown positioned between the circuitry 624 and the MRAM 628.The BL 604 is formed on top of the MRAM 628, as discussed relative toprior embodiments herein. While not shown in FIG. 6 due to the nature ofthe figure being a cross section view, the WL 608 is coupled to theaccess transistor in the circuitry 624.

An embedded MRAM, as those disclosed in various embodiments of thepresent invention, allows for fast and non volatile memory, and can beembedded on the same chip next to a microprocessor with much smallercell size. What makes an MRAM ideal for this application is that the MTJof the MRAM is fabricated on top of metals, with least interference withthe CMOS process. As discussed above, the MTJ elements are added laterwhen the micro processor is completed. In some embodiments of thepresent invention where spin torque transfer is used, the MTJ resistancechanges by spin current induced magnetization switching and uses verylittle current to do this unlike the field switching technique thatrequires large amount of currents.

Although the present invention has been described in terms of specificembodiments, it is anticipated that alterations and modificationsthereof will no doubt become apparent to those skilled in the art. It istherefore intended that the following claims be interpreted as coveringall such alterations and modification as fall within the true spirit andscope of the invention.

What is claimed is:
 1. A method of manufacturing a magnetic randomaccess memory (MRAM) cell comprising: forming a plurality of diffusionregions and a transistor gate, collectively defining an accesstransistor, in a silicon substrate; forming a plurality ofmetal-interposed-in-interlayer dielectric (ILD) layers on top of a firstILD layer with each of the plurality of ILD layers being formed on topof a previous layer of the plurality of ILD layer, each of the pluralityof ILD layers having metal dispersed therethrough, on top of the accesstransistor, the first ILD positioned on top of the diffusion regions;forming a magneto tunnel junction (MTJ) substantially on top of theplurality of ILD layers, each of the plurality of ILD layers having themetal dispersed therethrough, a bit line positioned in close proximityto the MTJ; forming a spacer on top of and around the MTJ; anddepositing a metal on top of the bit line and over a portion of the topmost one of the plurality of ILD layers and extending over the top ofthe MTJ.
 2. The method of manufacturing a magnetic random access memory(MRAM) cell, as recited in claim 1, wherein the spacer is made ofsilicon nitride.
 3. The method of manufacturing a magnetic random accessmemory (MRAM) cell, as recited in claim 1, wherein the spacer isapproximately 1,500 Angstroms in thickness.
 4. The method ofmanufacturing a magnetic random access memory (MRAM) cell, as recited inclaim 1, wherein the spacer is made of oxide.
 5. The method ofmanufacturing a magnetic random access memory (MRAM) cell, as recited inclaim 1, wherein more than one embedded MRAM is formed on the siliconsubstrate.
 6. A method of manufacturing a magnetic random access memory(MRAM) cell comprising: forming an access transistor on top of a siliconsubstrate; forming an embedded MRAM on a plurality ofmetal-interposed-in-interlayer dielectric (ILD) layers, each of theplurality of ILD layers including metal dispersed there through andincluding a first magneto tunnel junction (MTJ) is formed on top of ametal formed in the ILD layers that is in close proximity to a bit line,an MTJ mask is used to pattern the MTJ and is etched to expose the MTJ,ultimately, metal is formed on top of the bit line and extended tocontact the MTJ.